Vehicles such as, e.g., self-propelled construction machines, having a hydrostatic drive are generally exposed to extreme fluctuations with regard to the load to be handled and with regard to the vehicle speed to be realized. Frequent and rapid braking of the vehicle is therefore necessary. In particular, the wear of the conventional brakes is therefore relatively high. In the case of vehicles having hydrostatic drives, the vehicle is often braked by an infinitely variable change in the transmission ratio. However, the rotational speed of the internal combustion engine providing the requisite drive power is increased in order to absorb the kinetic energy of the vehicle during the braking operation. This increase in the rotational speed is detrimental to the service life of the internal combustion engine and in addition leads to generally unacceptable high noise emissions.
A common hydraulic circuit construction design for working machines is, e.g., known from WO 2009/084853 A2.
U.S. Pat. No. 6,644,025 B1 shows a control arrangement which is used to supply at least two hydraulic consumers with pressure medium.
An electric oil pressure system of a construction equipment is known from WO 2009/084853 A2.
US 2008/0256939 A1 shows a device for managing the displacement of a hydraulic motor or a group of hydraulic motors.
U.S. Pat. No. 6,688,419 discloses another vehicle hydraulic driving system including a closed hydraulic circuit with a hydraulic pump and a hydraulic motor. A leakage fluid outlet opening of the hydraulic motor and a valve arrangement are connected with a release chamber. In the release position of the braking control valve, the valve arrangement maintains a pressure in the release chamber corresponding at least to the release pressure of the braking device.
US 2010/0050620 A1 refers to a method of braking a hydrostatic drive machine. This method includes the steps of reducing a displacement of a pump of a hydrostatic drive system to a non-zero displacement, and increasing a displacement of a motor of the hydrostatic drive system to a displacement that is less than a maximum displacement. In a further step, an engine of the hydrostatic drive system is accelerated toward a desired engine speed range.
U.S. Pat. No. 5,946,910 shows a hydraulic circuit of a hydraulically driven working vehicle. A hydraulic circuit has a hydraulic travel pump and a working machine hydraulic pump which are driven by the power of an engine for discharging pressurized oil to a HST travel circuit and a working machine-driving hydraulic circuit, respectively. Pressurized oil from the working machine hydraulic pump joins pressurized oil from the hydraulic travel pump to effect high-speed travelling, while pressurized oil from the hydraulic travel pump joins pressurized oil in the working machine hydraulic pump to generate a large digging force to effect digging. The hydraulic circuit including the hydraulic travel pump and the working machine hydraulic pump is configured as an open hydraulic circuit.
Another hydrostatic drive including a hydraulic pump and at least one hydraulic engine is shown in U.S. Pat. No. 7,607,298 B2. The hydraulic engine is contacted to a closed circuit via a first working line and a second working line. In order to limit the swept volume flowing in one of the two working lines towards the hydraulic pump, a volume flow divider is provided in the working line and allows to remove a partial volume flow from the working line.
U.S. Pat. No. 6,338,247 B1 shows a hydraulic vehicle drive having a closed hydraulic circuit consisting of a variable displacement pump and one or several hydraulic motors. The variable displacement pump is driven by a diesel engine. During the braking or pushing operation of the vehicle, the hydraulic motor operates as pump and drives the variable displacement pump which, in turn, acts as a motor upon the diesel engine and acts to accelerate the latter. Thus, the intended braking effect prevents the danger that the diesel engine will reach non-permissible high revolutions per unit time, which is particularly dangerous for diesel engines with turbochargers. In order to prevent non-permissible high revolutions per unit time, a pressure regulator acting as a throttle is placed in the return line between the hydraulic motor and the variable displacement pump. The valve throttles the rate of flow when the pressure of the hydraulic fluid in the line exceeds a certain desired value during the braking operation.
A very similar system for controlling a hydraulic vehicle drive is shown in U.S. Pat. No. 6,360,537 B1. Here, braking takes place in a control member by a sequence valve being arranged in a line between the hydraulic motor and a variable displacement pump. Again, the hydraulic pump, the hydraulic motor, and the sequence valve are arranged in a closed hydraulic circuit.
U.S. Pat. No. 5,946,910 shows a hydraulic circuit for a hydraulically driven working vehicle which allows the vehicle to travel at an almost constant speed during high-speed travelling. The shown hydraulic circuit has a hydraulic travel pump and a working machine hydraulic pump which are driven by the power of an engine for discharging pressurized oil to an HST travel circuit and a working machine driving hydraulic circuit, respectively. Pressurized oil from the working machine hydraulic pump joins pressurized oil from the hydraulic travel pump to effect high-speed travelling, while pressurized oil from the hydraulic travel pump joins pressurized oil in the working machine hydraulic pump to generate a large digging force to effect digging. Again, all hydraulic circuits are designed as open hydraulic circuits.
GB 2 275 761 A refers to a vehicle with a hydrostatic drive. During braking of such a vehicle, an adjustable hydraulic motor of a hydrostatic propulsion drive is adjusted in the direction of the greatest possible capacity and a kinetic energy of the vehicle is partly directed into a prime moved and partly dissipated by pressure limiting valves in the closed working circuit of the hydrostatic drive. Accordingly, the hydrostatic circuit fluid is directed from the high pressure to the low pressure side via one or more pressure limiting valves. Thus, the kinetic energy of the vehicle is dissipated both by increasing the rotary speed of the prime mover and by throttling at the pressure limiting valve, the major part of the kinetic energy being converted at the pressure limiting valve into heating of the fluid.
If a construction machine as, e.g., a fork stacker, has no conventional service brake, the hydrostatic traction drive is utilized as a brake. Thus, all the kinetic energy of the vehicle, during braking or deceleration of the latter, is directed via the hydrostatic transmission to the internal combustion engine, which typically is a diesel engine. Since the internal combustion engine only has limited power absorption, a large proportion of the vehicle energy is transmitted into the flywheel, so that the rotational speed increases considerably. The problem described occurs in relation to the vehicle speed, its mass and the requisite braking deceleration.
It has therefore become known to provide a retarder drive, by means of which additional energy can be converted into heat. In wheeled loaders and off-highway stackers, because of considerably higher travel speeds to be achieved, conventional service brakes are also provided. In addition to the conventional service brakes, a retarder device, which works in parallel with the service brake, is provided in these vehicles. The retarder device serves in principle to reduce the brake wear and enables the vehicle to travel a longer distance on a gradient without reaching the critical temperature of the service brake. A feature common to the existing systems during the function of the retarder is a purely mechanical drive branch, so that the kinetic energy of the vehicle can pass to the variable displacement pump.
A principal object of U.S. Pat. No. 6,202,783 B1 is to provide a vehicle having a permanent hydrostatic traction drive wherein a retarder valve device provided instead of or in addition to a brake device, may work essentially free of wear, the internal combustion engine may be not overloaded, and the number of components for this arrangement may be reduced. The known hydrostatically driven vehicle has a variable displacement hydraulic pump and a hydraulic motor in a closed circuit. An internal combustion engine for driving the hydraulic pump and a variable displacement pump producing a volumetric output flow are connected in an open circuit. A pressure relief valve for avoiding a pressure in the open circuit above a maximum operation pressure is permanently connected to a hydraulic supply line to the hydraulic consumers. A retarder valve arranged in the open circuit is intended for the hydraulic absorption of braking energy. The retarder valve has a valve and a pressure limiting valve connected thereto, for throttling the volumetric output flow of the variable displacement pump. The variable displacement hydraulic pump, the variable displacement pump and the retarder valve are controlled by a digital electronic unit for the respective driving and working functions.
The retarder valve of U.S. Pat. No. 6,202,783 B1 including a valve and a pressure limiting valve may increase the overall manufacturing costs; particularly the costs for the valve and the associated pressure limiting valve additionally provided to the pressure relief valve permanently connected to the hydraulic supply line may be costly. In addition, the closed loop control system including the retarder valve unit may be prone to oscillations.
GB 2 205 632 A shows a drive device for driving a fork lift truck. The drive device comprises a primary energy source, a hydrostatic transmission and a pump for charging consumers with hydraulic energy. A deliberately actable valve is connected between the pump and the consumer. If braking occurs via the hydrostatic transmission and as a result the speed of the internal combustion engine is driven up, the pump is swung out to full delivery current. The pump delivers said full delivery current against pressure to which a pressure limiting valve is set. Delivery current is drained to this pressure limiting valve into a container with the consequence that the energy converted in the pump to generate the full delivery current against the pressure of the pressure limiting valve is converted as additional braking energy. Due to fact that the pressure limiting valve is connected in parallel with switching valves, none of the consumers can be charged with a higher pressure than the pressure to which the pressure limiting valve is set. The close circuit of this drive device includes two non-variable hydraulic motors. Accordingly, additional braking energy can only be obtained by swinging out the pump. Accordingly, the additional breaking energy, which can be obtained, is limited.
A hydraulic drive system without a separation in open and closed circuits is shown in US 2010/0050620 A1.
DE 103 04 917 A1 shows a similar hydrostatic transmission.
U.S. Pat. No. 6,360,537 B1 describes a system for controlling a hydraulic vehicle drive comprising solely a closed circuit without any consumers.
DE 2 142 946 refers to a hydrostatic system which is not usable as a hydrostatic drive system.
U.S. Pat. No. 6,339,928 B1 describes a system for controlling a hydraulic propulsion drive. Here, open and closed circuits are hydraulically connected with each other. The open circuit only serves to compensate any leakage of hydraulic fluid
The present disclosure is directed, at least in part, to improving or overcoming one or more aspects of the prior systems.